Conventionally, there has been proposed an image pickup apparatus which eliminates the need for a secondary optical system for detecting a focus by changing optical characteristics of a part of pixels in an image pickup device for imaging an object differently from optical characteristics of other pixels to be used for detecting a focus (See Patent Literature (PTL 1)). In the proposal, phase difference sensors in two kinds are arranged in a part of pixels in the image pickup device.
FIG. 4 is a diagram showing an example of arrangement of pixels in the image pickup device including the phase difference sensors for detecting a focus in specific rows.
In FIG. 4, reference characters R, G, and B respectively denote pixels in which a red, a green, and a blue filter are arranged on a light incidence surface thereof. Reference characters S1 and S2 denote a first and a second phase difference sensor which are different in optical characteristics from each other, used for detecting a focus, and arranged in the image pickup device along with the R, G, and B pixels to obtain color image signals.
FIG. 5 is a diagram schematically showing the structure of the first phase difference sensor S1. In FIG. 5, the first phase difference sensor S1 includes a micro lens 501 arranged over a planar layer 502. The first phase difference sensor S1 further includes a light shielding layer 503 and a photoelectric conversion element 504 which are arranged under the planar layer 502. The light shielding layer 503 has an opening eccentric from the center portion of a photoelectric conversion area of the pixel in a direction to one side (to the left side in FIG. 5) and has a diaphragm function to converge incident light.
FIG. 6 is a diagram schematically illustrating the structure of the second phase difference sensor S2. FIG. 6 is different from FIG. 5 in that the opening of the light shielding layer 503 of the second phase difference sensor S2 is arranged in a position symmetrical with respect to the opening of the light shielding layer 503 of the first phase difference sensor S1 through the center of an optical axis.
In FIG. 4, the more the number of pixels, the more similar the images formed in the row including the first phase difference sensor S1 and the row including the second phase difference sensor S2. When object light is focused on a pixel through an image pickup optical system, the image signals coincide with each other in the row including the first phase difference sensor S1 and the row including the second phase difference sensor S2.
When the object light is out of focus, a phase difference arises between the image signals in the row including the first phase difference sensor S1 and the row including the second phase difference sensor S2. The phase shift arises reversely according to the case where the object light is out of focus toward the front or the rear of a camera. When the image pickup optical system is viewed from the first phase difference sensor S1 and the optical system is viewed from the second phase difference sensor S2, the pupil is seen as if it were divided symmetrically with respect to the center of the optical axis.
FIGS. 7A and 7B are schematic diagrams useful in explaining a phase shift of an image being out of focus. It should be noted that in FIGS. 7A and 7B, the first and the second phase difference sensor S1 and S2 are abstractly made close to each other to be indicated by points A and B. For the sake of easy understanding, the R, G, and B pixels for picking up an image are omitted from the illustration in FIGS. 7A and 7B to show as if the phase difference sensors are aligned.
Light from a specific point of an object is separated into pencil of light rays (ΦLa) passing through the pupil corresponding to A and being incident on the corresponding A and pencil of light rays (ΦLb) passing through the pupil corresponding to B and being incident on the corresponding B. The two pencils of light rays are derived from one point, so that the two pencils of light rays reach one point to which the two pencils of light rays are converged by the same micro lens if the focus of the image pickup optical system is on the surface of the image pickup device (refer to FIG. 7A). However if a focus is at a position of a distance x, for example, in front of the surface, two pencils of light rays are shifted from each other in accordance with a change in incident angle of light (refer to FIG. 7B). If the focus is at a position of a distance×deep therefrom, two pencils of light rays are shifted in a direction opposite to a direction in which the two pencils of light rays are shifted in the former case.
For this reason, if the image pickup optical system is focused, the image signals generated by the arrangement of A and B coincide with each other. If not, the image signals are shifted.
However the image pickup device including the phase difference sensor will lack pixel data corresponding to the position of the phase difference sensor at the time of capturing a still image. When the signal obtained by the phase difference sensor is used as an image signal for a still image, a different visual field lacks continuity between peripheral pixel signals, which looks like a flaw.
To solve such a problem, in the above Patent Literature 1, the image signal corresponding to the position of the phase difference sensor is interpolated by the image signal of peripheral pixels.
In an arrangement of pixels in the image pickup device shown in FIG. 4, interpolation data are inserted into the portions of the phase difference sensors S1 and S2 included in the picked-up image signal from peripheral pixels. In FIG. 4, the R, G, and B pixels used for picking up an image are arranged in a Bayer array and the phase difference sensors S1 and S2 are arranged instead of a part of pixels G. Data of the pixel G lacked due to the existence of the phase difference sensors S1 and S2 is created into a combined pixel G data using data of four pixels G diagonally adjacent thereto and the combined pixel G data is applied to the data of the lacked pixel G.
There has also been proposed an image pickup apparatus which changes over interpolation correction, offset correction and gain correction process according to the defect level of a defect pixel (See Patent Literature (PTL 2)).